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Avalanche Forecast

Archived

Mar 9th, 2017–Mar 10th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

Storm loading will promote touchy conditions on Friday. Expect the avalanche danger to increase throughout the day.

Confidence

Moderate - Timing or intensity of solar radiation is uncertain on Saturday

Weather Forecast

Thursday night: 10-15cm of new snow / Moderate southwest winds / Alpine temperature of -8Friday: 12-18cm of new snow / Strong to extreme southwest winds / Alpine temperatures of -3 (freezing levels at 1500m)Saturday: Clear skies in the morning with increased cloud in the afternoon / Light southwest winds / Alpine temperature of -3Sunday: Light flurries / Light southwest winds / Alpine temperature of -4

Avalanche Summary

Over the past few days, natural and human-triggered storm slab and loose dry avalanches to size 2 were reported on a variety of aspects. Cornices are large and remain easy to trigger. A great image from Sunday illustrates just how surprising the results can be. Click here for details. A size 3.5 deep persistent slab avalanche also occurred on a northeast facing alpine slope on Mt Hosmer. The avalanche, which likely occurred on Wednesday, is thought to have been cornice-triggered and ran a distance of about 900m. This event highlights the isolated, yet destructive, nature of the deep persistent avalanche problem which should become more prevalent as we head into spring.Looking forward, natural storm slab activity is expected to ramp-up again in response to new snow and wind on Thursday night and Friday.

Snowpack Summary

By Friday morning expect 10-15cm of new snow with lots more falling throughout the day. Strong southwest winds are expected to shift these accumulations into much deeper deposits in exposed lee terrain. The overall storm total for the last week or so now stands at roughly 80-130 cm. At times, strong southwest winds have extensively redistributed the snow forming large brittle cornices. In some areas the recent storm snow is settling into a cohesive slab which sits over a variety of old surfaces including surface hoar, facets, stiff wind slabs, sun crusts, and a rain crust below 1900 m. Little is known about the reactivity of this interface which was buried in the middle of February. Until we get a better feel for it, I'd assume it has potential for large and destructive avalanches.Isolated basal facets still exist in shallow snowpack areas and can produce destructive full-depth avalanches. Likely triggers include intense storm loading, a cornice fall, or strong solar radiation.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.